Re: mysqldump usage in bash

[Arenstar]

Hello Greg,

So im currently using eval, based on the fact that
exec/bash does not see the > in the --where clause. (not the redirection)
It is currently working, though im open ears to you explaining how this
could be a problem.

What effects can eval have? that i am unaware of. In fact ive never used
eval before, it just wasnt neccessary..
Thank you for your interesting reply
 
query="mysqldump -h $DBSource -u rxxxxx -pxxxx $DB $TABLE --where '$Field >
$(($IDstart-1)) and $Field < $(($IDend+1))' > ./tmp/$TABLE$Dumpname"
 eval $query

Kind Regards
Arenstar


Greg Wooledge wrote:
> 
> On Thu, Aug 27, 2009 at 06:13:38AM -0700, Arenstar wrote:
>>     temp="mysqldump -h $DBSource -u $USER -p$PASS $DB $TABLE
>> --where='$Field
>> > $VarStart  AND $Field < $VarEnd' > $TABLE$DumpName"
>>     exec $temp
> 
> The obvious problem here is that you want the last ">" to be treated as
> a redirection operator.  This means the parser has to see it, outside
> of a variable, and recognize it as a redirection, rather than a piece
> of raw data.  Either the > has to be put on the exec line outside of a
> variable, or you to use eval.  But if you use eval, it would change the
> meaning of your inner quoted expression.
> 
> There is also a more subtle problem you've missed: the --where thing
> contains whitespace, so it's going to be word-split in ways that you
> simply can't deal with using this sort of approach.
> 
> Best bet if you have to retain the putting-commands-into-variables
> nonsense
> is to split it into two parts: a command, which should be stored in an
> array, rather than a string; and an output destination.
> 
>   command=(mysqldump -h "$DBSource" ... --where="$Field > $VarStart...")
>   output=$TABLE$DumpName
>   exec "${command[@]}" > "$output"
> 
> This accomplishes a few things:
> 
>  1) The command can have all the quoted whitespace you want without any
>     danger.  You just can't have any pipes, redirections, etc. in it.
>  2) The > is seen as a redirection, without problems.
>  3) You didn't have to use eval, and endure the headaches eval brings with
> it.
> 
> Now, personally, I'd just skip all the putting-commands-into-variables
> nonsense and write it like this:
> 
>   exec mysqldump -h "$DBSource" -u "$USER" -p"$PASS" "$DB" "$TABLE" \
>     --where="$Field $VarStart AND $Field < $VarEnd" \
>     > "$TABLE$DumpName"
> 
> I cover these issues in a bit more depth at:
> http://mywiki.wooledge.org/BashFAQ/050
> 
> 
> 
> 

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